1. Field of the Invention
The present invention relates to a surface acoustic wave device, and particularly to a surface acoustic wave (SAW) device usable for demodulation of data in a spread spectrum communication, and a demodulating circuit and a communication system using such a SAW device.
2. Related Background Art
The SAW is a wave which is propagated with its energy concentrated in the vicinity of a substrate surface, and its signal can be readily input or output through the substrate surface. Hence, the SAW device is noted as a signal processing device.
FIG. 1 illustrates a prior art SAW device. Its operation principle will be described with reference to FIG. 1. In FIG. 1, there are arranged, on a piezoelectric substrate 31, an input electrode 32 for exciting a SAW and an output electrode 33 for converting the SAW to an electric signal. The output electrode 33 is made up of a plurality of taps formed equidistantly on a SAW propagation path. Each tap consists of so-called a comb electrode, and comprises a pair or plural pairs of electrode digits. In the structure, when a signal is applied to the input electrode 32, a SAW corresponding to the signal is generated and propagated toward the output electrode 33. At the output electrode 33, the SAW reaches each tap at a constant interval. Therefore, an output is a time-sampled composite output. At each tap, a polarity can be altered by the arrangement of electrode digits. As a result, an output can be taken as a sum of values obtained by sampling the input signal at a constant interval and adding thereto the polarity of each tap. Therefore, a correlator operates by appropriately setting the polarity of each tap, and the output becomes maximum when the input signal coincides with the pattern of taps in the output electrode 33.
The SAW device can operate in the same manner even if the input and output electrodes are exchanged with each other, and in this case, the output becomes maximum when the input signal coincides with the tap pattern of the input electrode 32 of the SAW device.
FIG. 2 shows a prior art demodulator that uses the prior art SAW device illustrated in FIG. 1. In FIG. 2, a received signal is input into a prior art SAW device 20 through amplifier, filter, frequency changer and so forth. An output of the SAW device 20 is divided into two, and one of them is directly input into a multiplier 25 while the other one is input into the multiplier 25 through a delay circuit 26. If a signal modulated by a predetermined code series is received, a correlation peak appears as an output of the SAW device 20 when the modulated signal coincides with the tap pattern of the SAW device 20. Here, the tap pattern of the SAW device 20 coincides with a code series for one bit of the signal. Therefore, the correlation peak appears from the SAW device 20 for each one bit of the signal. Modulation information of the signal is stored in the correlation peak. Particularly, in the case of a phase modulation signal, phase modulation information is stored in a carrier wave of the correlation peak. The output of the SAW device is divided into two, and one of them is directly input into the multiplier 25 while the other one is input into the multiplier 25 after delayed by one bit of the signal. Thus, one-bit delayed detection is performed to demodulate the signal.
In the prior art SAW device as shown in FIG. 1, however, the tap pattern for one bit of the signal is only provided, and hence an external delay circuit and so forth are needed to compare adjacent bits with each other. Therefore, in the prior art demodulating circuit which uses such a prior art SAW device as shown in FIG. 2, loss in the delay circuit is large, and hence an amplifier needs to be inserted upstream and/or downstream of the delay circuit. Further, since a nonlinear clement such as a multiplier needs to be used, the size of an entire circuit is inevitably large. Moreover, quality of signal is lowered.